To enhance a statement mentioned by Lokked:- "...You might (WILL!) get r**** in (ALL) fights (IN THE END_GAME)..."
Stacking does not work in EQ2. (Please note the super-sized 'period')
Think of it this way, most mobs are designed by the developers under the assumption that a basic team will:
- be able to put out enough dps ('dps') to overcome a mob of similar level
- have appropriate buffs ('b') to either increase survivability or DPS ('dps')
- are able to debuff certain mob attributes and characteristics ('db')
- be able to take incoming damage considering both their level and the mob ('id')
- fights last, on average, for a certain amount of time ('t') thus finding the break even between id & dps (as the longer a fight lasts, the more chance both team hp and mana will decrease - therefore, the quicker a fight (+dps) potentially, the less id taken by the team plus faster recovery (mana, etc.)

By having a stacked team, you'll be missing out on a lot of these basic team building assumptions.

For example, as you mentioed, all paladin team:
- low dps = longer fights (t) = increase incoming (+id) = -hp & -mana = death spam
- same line buffs (b) will not stack so your potentially only running a team as if 1 person did all the buffing as opposed to 6 people running buffs which again = low dps (or team attributes / characteristics / resists) = longer fights (t) = increase incoming (+id) = -hp & -mana = death spam
- each class debuffs certain mob attributes or characteristics - for example, class 'x' may debuff the mobs cold resist and str whereas class 'y' may debuff a mobs heat resist and int. By running the single classes, your missing out on the ability to debuff entire lines of mob attributes and characteristics. As you can guess, once again you will experience low dps = longer fights (t) = increase incoming (+id) = -hp & -mana = death spam
- an all paladin team may potentially be able to take more incoming damage but seriously, paladins run on mana, once that's gone it will just be a slug fest which = ... can you guess ... longer fights (t) = increase incoming (+id) = -hp & -mana = death spam

In all honestesy mooglej - have a quick read of this entire thread - grab a team you like the look of and run with that - it will save you a lot of pain down the track